In recent years, there has been a demand for higher speeds in communications, and various attempts have been made to achieve higher speeds in communications. In order to reduce amounts of data in transmissions, for example, deduplication is performed on data that is a transmission target in some cases. In deduplication, a communication device on the reception side stores, in association with an identifier, data that it received from the communication device on the transmission side in the past, and the communication device on the transmission side also stores data that it transmitted in the past and the identifier in association with each other. The communication device on the transmission side determines whether or not part or all of the data as the transmission target is data that it has already transmitted, and for data that it has already transmitted, the communication device on the transmission side transmits, to the communication device on the reception side, the identifier that is associated with that data. The communication device on the reception side reads the data that is stored in association with the received identifier, and treats the read data as data transmitted from the communication device on the transmission side.
FIG. 1 explains an example of a transfer process in a case when deduplication is performed. When for example data d1 is transmitted, the communication device on the transmission side partitions transmission data into a plurality of pieces of data, and determines data that was transmitted in the past and data that is included a plurality of times in transmission data. In the example illustrated in FIG. 1, data d1, which is transmission data, includes data d2 and data d3 in an order of data d2, data d3, data d3 and data d2. Also, it is assumed that the communication device on the transmission side has transmitted neither data d2 nor data d3 to the communication device on the reception side. In such a case, the communication device on the transmission side transmits identification information ida that represents data d2 to the communication device on the reception side together with first data d2 in the transmission data. The communication device on the reception side stores data d2 and identification information ida in association with each other. In FIG. 1, identification information ida is denoted by A to facilitate understanding. Similarly, the communication device on the transmission side transmits identification information idb that represents data d3 to the communication device on the reception side together with data d3 in the transmission data. In FIG. 1, identification information idb is denoted by B. The communication device on the reception side stores data d3 and identification information idb in association with each other.
Next, the communication device on the transmission side transmits identifier idb (B) that represents data d3 to the communication device on the reception side instead of data d3 that follows in the transmission data. When receiving identification information idb, the communication device on the reception side determines data d3 stored in association with identification information idb, and reads data d3 in place of the reception data. Similarly, the communication device on the transmission side transmits identifier ida (A) that represents data d2 to the communication device on the reception side instead of data d2 that follows in the transmission data. When receiving identification information ida, the communication device on the reception side determines data d2 stored in association with identification information ida, and reads data d2 in place of the reception data. Thus, after transmitting data d2 and data d3 one time, just by transmitting the identification information of these pieces of data, the communication device on the transmission side can achieve the same effect as that achieved when data d2, data d3, data d3 and data d2 are transmitted to the communication device on the reception side. The communication device on the reception side restores data d1 by connecting data d2, data d3, data d3 and data d2.
As a related technique, a device has been suggested that assigns a pattern number to a pattern having a high appearance ratio to store it, and generates transfer data by replacing a pattern in data with its pattern number when the pattern included in data corresponds to a stored pattern (Patent Document 1 for example). The transmission device transfers, to the device on the reception side, information of transfer data and a pattern registered in a pattern dictionary. Further, a transfer method is known in which data to which a value indicating that a device on the data transmission side is performing conversion identification is added is transferred when the value of one byte of data that is a transmission target is a value used for a control signal. In this transfer method, data is not changed for values in a prescribed range that is different from control information (Patent Document 2 for example).                Patent Document 1: Japanese Laid-open Patent Publication No. 2002-368623        Patent Document 2: Japanese Laid-open Patent Publication No. H9-331366        